Choline hydroxide or choline base (e.g., 2-(hydroxyethyl)trimethylammonium hydroxide), is a strong base which has applications in the production of other choline salts, for example, by neutralization with an appropriate acid or in applications where a strong base containing very low levels of inorganic ions is needed or can be tolerated. For instance, a choline base, such as choline hydroxide, is important in applications, such as in manufacturing electronics.
Choline hydroxide may be manufactured in a variety of different ways. For example, choline hydroxide may be produced from choline halides by displacing the halide counterion with hydroxide, for example, by using Ag2O. Choline hydroxide may be formed by treating choline sulfate (which may be obtained by the reaction of dimethylamino ethanol and dimethyl sulfate) with Ba(OH)2 resulting in BaSO4 precipitate as coproduct.
Choline hydroxide may also be manufactured from choline halides. Choline halides (e.g., choline chloride) may be formed by the reaction of trimethylamine (TMA), ethylene oxide (EO), and HX, X being a halide, in one or more steps. Choline chloride may be converted to choline hydroxide by electrolysis in electrolytic cells having the cathode and anode separated by a cation exchange membrane (e.g., chloride ions are oxidized to Cl2 at the anode and water is split into hydroxide ions and hydrogen gas at the cathode). Choline cations pass through the cation exchange membrane and combine with the hydroxide ions to form choline hydroxide. Regardless of the method used, producing choline hydroxide from choline halide results in a halogen containing side stream.
Choline hydroxide may also be produced by the direct reaction of trimethylamine (TMA), water and ethylene oxide (EO). The direct synthesis may be performed in a suitable solvent, e.g., water or water miscible alkanols. The direct method has the advantage of being much more atom efficient than the above described methods involving a choline halide starting material. However, the direct reaction of EO and TMA in the absence of a strong acid (e.g., HX) also has some disadvantages that are mainly due to the nature of the choline hydroxide product. Because of the strong basic nature of choline hydroxide, the molecule is prone to side product formation via O-ethoxylation and to color formation and degradation, for example, due to Hofmann elimination during synthesis.
As choline hydroxide has a similar base strength as NaOH, it is able to activate its own hydroxyl groups, resulting in an important competition between N and O-ethoxylation. In the case of N-ethoxylation a free amine (TMA) reacts with an ethylene oxide molecule, resulting in the desired choline molecule. In the case of O-ethoxylation, the hydroxyl group of a choline molecule reacts with another EO molecule resulting in choline like molecules with a higher degree of ethoxylation. O-ethoxylated products represent impurities in the final product. Furthermore, in many applications (e.g., production of various choline salts) the molarity of the hydroxide ion is important and therefore each molecule of EO spent on O-ethoxylation represents an economical loss. The degree of O-ethoxylated products that is observed during choline hydroxide synthesis may be dependent on the base strength of the solution, and hence upon the hydroxide (i.e., choline hydroxide) concentration. Indeed, the amount of O-ethoxylated products in a 10% aqueous reaction mixture is virtually zero, while in a commercial 45% choline hydroxide solution, the amount of higher choline ethoxylates can be as high as 10% by weight and higher. Apart from the concentration, O-ethoxylation is also enhanced by higher reactor temperatures.
Furthermore, choline hydroxide is known to be unstable and to develop color during synthesis and storage due to decomposition. Decomposition may occur via a so-called Hofmann elimination, resulting in the formation of TMA and acetaldehyde. Acetaldehyde ultimately leads to heavily colored condensation products, causing concentrated choline hydroxide solutions to become brown and black in a matter of a few days at room temperature. Hofmann elimination reactions are favored by higher temperature, and the temperature must be kept low during the synthesis of choline hydroxide in order not to obtain product already heavily colored immediately after preparation.
Thus, there remains a need for an effective and efficient process for producing choline hydroxide without undesired by-products and color formation.